Water-absorbing resin (super absorbent polymer [SAP]) is a water-swellable, water-insoluble polymer gelling agent. Water-absorbing resin is used in various applications including use in absorbent articles such as disposable diapers and sanitary napkins, agricultural and horticultural water retaining agents, and industrial waterproofing agents.
In recent years, these sanitary products such as disposable diapers, sanitary napkins, and incontinence pads have become higher in functionality and thinner, so that a larger amount of water-absorbing agent tends to be used in a single sanitary product, and a water-absorbing agent tends to be contained in a larger amount with respect to the entire absorbent body that is made of, for example, a water-absorbing agent and hydrophilic fiber. Specifically, studies have been carried out on, with use of a smaller amount of hydrophilic fiber with a small bulk specific gravity and a larger amount of a water-absorbing agent with an excellent water absorbency and a large bulk specific gravity, reducing the thickness of a sanitary product without reducing the amount of water absorption by increasing the proportion of a water-absorbing agent in an absorbent body.
Though such a sanitary product, in which the content of a water-absorbing agent is increased by decreasing the proportion of hydrophilic fiber, tends to be preferable from the viewpoint of simply storing a liquid, a problem arises to the contrary in consideration of distribution and diffusion of a liquid in actual use of the sanitary product.
A water-absorbing agent that absorbs water turns into a soft gel-like water-absorbing agent. Thus, in a case where such a water-absorbing agent that is large in amount per unit volume has absorbed water, a gel blocking phenomenon occurs. This phenomenon causes a considerable reduction in diffusibility of a liquid in a sanitary product. As a result, that part of the water-absorbing agent which is distant from a central area of the sanitary product and which is therefore difficult for the liquid to reach does not effectively function. This prevents the effect of increasing the water-absorbing agent content from being sufficiently exhibited, so that the sanitary product in actual use has an absorbing capability that is much lower than the theoretical level.
In order that such a problem is avoided and the absorbing capability of an absorbent body is maintained, the ratio between hydrophilic fiber and a water-absorbing agent is inevitably restricted, so that the thinning of a sanitary product is also limited.
Examples of indexes used to evaluate an reduction of gel blocking in a sanitary product include a fluid retention capacity under load (absorbency against pressure [AAP] or performance under pressure [PUP]), indicative of a water absorbing property under load, and a saline flow conductivity (hereinafter abbreviated as “SFC”; see Patent Literature 1).
As publicly known techniques for reducing gel blocking, there have been known the following techniques: a technique of changing the crosslinking densities inside and outside of a water-absorbing agent by a surface treatment; a technique that combines a surface treatment with inorganic compound(s) as a liquid permeability improving agent such as inorganic microparticles and a polyvalent metal salt and/or cationic polymer compound(s) as a liquid permeability improving agent; a technique for improving water absorption performance, particularly liquid diffusibility; and a technique for controlling a reaction environment of a surface-crosslinking treatment (see Patent Literatures 1 to 39).
Recent years have also seen a tendency of an increase in the use of disposable diapers in developing countries. In such less rich countries, there may be an increasing demand in the future to wash a disposable diaper, used once, with water lightly and use it again from the viewpoint of reducing the cost of purchasing diapers. Further, developed countries now have aging society. Elderly people in recent years have a larger proportion of drinkers than in the past. Among drinkers, those who wear a diaper habitually may first discharge urine that is thin like water and then discharge normal urine.